Spark-ignition engines being built and offered at present have a relatively low compression as compared with a diesel engine, and therefore have a higher fuel consumption especially in the part-load range. The diesel engine, on the other hand, requires a relatively stronger and heavier construction because of its high peak pressure, generally runs louder and more roughly, and generates more nitrogen oxides but, especially, more health-endangering soot particles.
The higher part-load fuel consumption of the spark-ignition engine has an effect principally in part-load operation, which is very negative for everyday operation since our automobiles, because of heavy road traffic, can now be operated almost only in the part-load range.
A variety of improvements have been introduced for lowering the fuel consumption of the spark-ignition engine: improving combustion by way of swirl and turbulence in the fuel-air mixture, mixture leaning, charge stratification, multiple sparking, and others. In the case of the diesel engine, improvements have been sought by way of direct fuel injection into precombustion chambers and secondary combustion chambers, a directed injection stream, and much more. All the improvements in the prior art have not, however, yielded any substantial decrease in fuel consumption, especially in the spark-ignition engine.
In order to appreciably lower fuel consumption in the spark-ignition engine, especially in the part-load range, the compression ratio must be effectively raised, so that thermal efficiency can increase appreciably. Present-day spark-ignition engines operate, as is known, with structural compression ratios of approx. ε=10:1. If this value is substantially exceeded, the risk then exists, during full-load operation, of spontaneous ignition with “pinging” or even “knocking” combustion, which, as is known, can result in a drastic drop in torque and power and possibly in destruction of the combustion engine. Diesel engines operate with structural compression ratios of ε=18 to 25:1, including in the part-load range, since the diesel engine, in contrast to the Otto-cycle engine, must compress almost the entire cylinder charge in every load condition, in order to achieve the necessary self-ignition temperature. The diesel engine therefore operates, particularly in the part-load range, with a substantially higher thermal efficiency than the spark-ignition engine, which, because of its throttle control system, compresses only a partial charge in each case and thus sometimes operates at very low effective compression ratios of less than ε=3:1. At idle, for example, a spark-ignition engine has only an approx. 20 to 25 percent charge.
A disadvantage of the diesel engine, however, is that because of the high compression of almost the entire cylinder charge, the engine must also perform a great deal of compression work. It is therefore known in the art that a spark-ignition engine that could operate in the part-load range with high compression ratios similar to those of a diesel engine would be superior to the latter because of the lower compression required.